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Re: Elevated post-tensioned slab cracks[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Elevated post-tensioned slab cracks
- From: GSKWY(--nospam--at)aol.com
- Date: Sat, 11 Jan 2003 14:24:01 EST
With respect to some of the other posts that have addressed this question:
To be honest, I am never really quite sure what people are talking about when they refer to the "catenary action" and "catenary capability" of either tendons or nonprestressed integrity steel. Or more exactly, it seems like it is used in different way .
My kind of fuzzy impression is that since the word catenary has some connection with chain, if the concrete is cracked, either tendons or nonprestressed steel are able to keep the concrete together somewhat, like beads on a chain. Although I may be completely wrong, and it might mean something completely different in Australia.
With respect to the discussions on one- vs. two-way failure, I would guess you are actually a long ways from failure. In fact there has never been a post-tensioning failure due to any kind of slab failure that I am aware of. The l'Ambiance Plaza failure was due to failure of the lift-slab connections, the Skyline Parking garage failed because the building next door fell on top of it and the Berlin Congress Hall failed because the type of prestressing steel the Germans used is apparently very susceptible to stress corrosion cracking.
I have seen two-way slabs with a very large number of broken tendons and lots of cracking but I have never seen "failure". In general I would say that two-way post-tensioned slabs seem to have very great factors of safety - I have also seen slabs with alot of broken tendons but no cracking.
In my humble opinion, you don't really need to be worrying about whether you will have a one- or two-way failure - what you do need to figure out is where the water is coming from and what tendons might have water in them. Try to get the water out. Also do some investigation to look for broken tendons - I would be surprised if you found any though, at least any broken due to corrosion.
I would note again though that it is somewhat risky to opine on cracks one hasn't seen, particularly if one hasn't read the original post closely.
The most spectacular cracking I have seen is in a situation somewhat similar to yours - three underground parking levels (two-way post-tensioned) for a condo building in Northern Virginia. Some of the cracking was due to restraint but most of it was due to blatant and almost unbelievable incompetence by the design engineer. (Whose response to my report that there were problems was "All concrete cracks".)
Crack distribution is sometimes a function of the geometry of the structure. I.e. most one-way (slab and beam) structures tend to be a lot longer in the direction of the distributed tendons . The beam direction might be 60 + 20 = 80 or 60+20+60 = 140ft, the slab span direction might be 250. Most of the restraint cracking will be parallel to the beams.
I am guessing that the partial step you are referring to is a one ft or so offset that allows for a drainage slope. Since you have tendons anchored at different elevations, you have a little couple. The torsion is making the cracking in that area a little worse.
Gail S. Kelley
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